Device for producing three-dimensional models

ABSTRACT

The present invention relates to a device for manufacturing three-dimensional models by means of a 3D printing process, whereby a build platform for application of build material is provided and a support frame is arranged around the build platform, to which said support frame at least one device for dosing the particulate material and one device for bonding the particulate material is attached via the guiding elements and the support frame is moveable in a Z direction, which essentially means perpendicular to the base surface of the build platform, said movement provided by at least two vertical positioning units on the support frame. In this respect, it is provided that the positioning units are respectively separate components and are arrangeable on the support frame independently from one another and the location and orientation of such can be adjusted independently from one another.

The invention relates to a device for manufacturing three-dimensionalmodels by a 3D printing method as expressed in the generic concept ofpatent claim 1.

A method for producing three-dimensional objects from computer data isdescribed in the European patent specification EP 0 431 924 B1. In thismethod, a particulate material is deposited in a thin layer onto aplatform which, if needed, is surrounded by a chamber and then a bindermaterial is selectively printed according to computer data on theparticulate material using a print head. The particle area onto whichthe binder is printed sticks together and solidifies under the influenceof the binder and, if necessary, an additional hardener. The platform isthen lowered by a distance of one layer thickness into a build cylinderand provided with a new layer of particulate material, which is alsoprinted as described above. These steps are repeated until a certain,desired height of the object is achieved. A three-dimensional object isthereby produced from the printed and solidified areas.

The object produced from the solidified particulate material asdescribed above is embedded in loose particulate material and such issubsequently removed therefrom. This is done, for example, using anextractor. This leaves the desired objects, from which the remainingpowder is removed, e.g. by brushing.

Other powder-supported rapid prototyping processes work in a similarmanner, for example, selective laser sintering or electron beamsintering, in which a loose particulate material is also deposited inlayers and selectively solidified with the aid of a controlled physicalradiation source.

All these methods are referred to collectively below as“three-dimensional printing methods” or “3D printing methods”.

However, the provision of a build container with the verticallylowerable build platform located within requires a high technical effortin respects to sealing of the chamber wall against the platform toprevent the particulate material from uncontrollably flowing out throughthe gap between the edge of the build platform and the chamber wall,otherwise the danger exists that the platform will jam against thechamber wall due to the possibly grainy particulate material.

Another disadvantage of the lowerable build platform is the constantlyincreasing weight on the build platform to be moved as the buildingprocess progresses. In particular during application of the new layer,it may be necessary to lower the powder bed by more than the layerthickness and then raise it again to the dimension required in order toadjust the layer thickness with sufficient accuracy. In the case of sucha revolving operation, not only the total weight of the powder feedstockincluding the build platform must be overcome, but also the frictionalforces of the powder bed relating to the chamber walls and the sealfriction between the build platform and chamber wall. This results inhigh loads for the guides and drives, especially when dealing with largebuild spaces and high feedstock thicknesses of the particulate materialemployed.

In this regards it is known from the prior art of EP 0 644 809 B1 and DE10 2006 030 350 A1 of a method whereby the particle bed is not loweredrelatively to the worktable, but rather the coating apparatus and theprint head used for applying the particulate material and binder areraised relative to the particle bed.

Thus it is known from the prior art that a coater for particulatematerial and a print head are attached to a support frame and thereuponarranged. The guiding elements are thereby integrally designed in total,for example, by connection with one another via a base plate. Inparticular, this is done to achieve good distribution of force.

Furthermore, as is known from the prior art, four Z axes are providedand all Z axes are connected with one another via connecting rods on theupper side and connecting rods on the underside. The manufacturingaccuracy of the dimensions required for such sizes is very imprecise andtherefore the system is absolutely statically over-defined. At the sametime, it is expensive and difficult to install.

Since such designs normally require welded constructions, a Z axlestructure must be adapted for almost all sides. Since the accuracyrequirements are, however, quite high, such a welded construction isvery expensive and very difficult to get.

According to the present invention, an object of the device relates toenabling the previously mentioned art to obtain exact Z axis movement ofboth the coater of particulate material and the print head that issimple to install and adjust.

This object is achieved by a device according to patent claim 1.

According to the present invention, a device for manufacturingthree-dimensional models by means of a 3D printing process is described,whereby a build platform for application of build material is providedand a support frame is arranged around the build platform, to which saidsupport frame at least one device for dosing the particulate materialand one device for bonding the particulate material are attached via theguiding elements, and the support frame is moveable in a Z direction,which essentially means perpendicular to the base surface of the buildplatform.

According to the present invention, the positioning units arerespectively separate components and are arrangeable on the supportframe independently from one another and the location and orientation ofsuch can be adjusted independently from one another.

According to a preferred embodiment of the present invention, a printhead for dosing liquid droplets is suggested as a device for bondingparticulate material. When brought in contact with the particulatematerial, this liquid leads to locally confined bonding. However, thereare also other devices for bonding that could be employed, such assystems for the creation of high energy beams, for example lasers, thatin turn lead to locally confined bonding of the particulate material atthe point of impingement.

Preferentially the support frame displays an essentially rectangularshape when viewed from a top view. This may be advantageous due to thefact that 3D printing systems with linearly arrayed print heads obtain ahigher surface print speed if the surface to be printed is rectangularlyshaped and the print head prints across the long side. Moreover, as thewidth increases, the difficulty to manually remove unbonded particulatematerial from the structural body also increases. Therefore, with thisin mind, we also recommend limiting the width to a manageable size. Anarea of 1 to 2 m can be named as a practical size. In such a case, theloose particulate material can be manually removed with simple cleaningaids. Larger dimensions are feasible, but they require more elaborateaids, such as walkable bridges or machine-assisted cleaning systems.

The suspension points of the support frame to the positioning units arehereby preferentially arranged at points other than the corners. In thecase of larger dimensioning of the support frame, it may proveadvantageous to provide additional suspension points.

It has been shown that a high rigidity and stability against vibrationcan be achieved by suspending the support frame via two suspensionpoints arranged on each of the two long sides of the rectangular supportframe.

In so doing, the positioning units preferentially can be adjustedindependently from one another in terms of location and orientation and,yet more preferentially, each positioning unit engages with the supportframe at a suspension point and is independently moveable in the Zdirection.

Furthermore, it may prove advantageous if at least two verticalpositioning units are connected via a supporting structure. In so doing,the supporting structure may also be flexibly deformable.

In so doing, it may be provided that every supporting structure and/orevery vertical positioning unit has at least three vertically adjustablefeet.

The device can be such that, for example, it is constructed without aseparate foundation due to the fact that forces are optimallytransferred to the floor. This can be accomplished by means of e.g. afixed connection of the Z axis with the floor via machine feet that areadjustable in the X, Y and Z directions.

For this purpose, Z is the vertical direction, X is the horizontaldirection of the longer side of the support frame and Y corresponds tothe short side of the support frame.

According to the invention, few requirements are placed on the floor. Itmust be able to safely bear the weight of the machine without deformingseverely. No special demands are placed on the levelness or surfacequality. A concrete floor constructed to the current state of technologyis adequate for the intended purpose.

According to one preferred embodiment of the present invention, thesuspension points are the Bessel points of the support frame. Such anembodiment achieves the statically lowest flexing of the support frame.

In order to achieve high accuracy and high stability while maintaininglow weight, the support frame is preferably formed from rectangular tubeprofiles made of steel. Other design shapes and materials, for instance,fibre-reinforced composites, are likewise feasible without compromisingthe functionality.

Preferably, as already described, every suspension point can beindependently moved in the Z direction by a positioning unit.

Such a vertical positioning unit could consist of e.g. a linear thrustunit, preferentially having a linear actuator, a drive motor and alinear guide.

To ensure movement of the vertical positioning units that is as uniformas possible, the drive motors are, according to a preferred embodimentof the present invention, coupled to one another via an electronicgantry system.

According to another preferred embodiment of the present invention,every vertical positioning unit is held by its own, stand-alone axleframe. In such a way, it is possible to achieve a modular constructionof the entire device. The vertical positioning units can thereby beadvantageously and independently of one another exchanged without aproblem.

In order to achieve high accuracy and high stability while maintaininglow weight, the weight-bearing structure of every vertical positioningunit is preferably formed from light metal plates which are pinnedand/or screwed. This offers the advantage that the spatial structureconsists of relatively simple, flat surface planes, which can be workedwith in one work step.

Material distortion due to welding or retrofitting work does not occurand therefore there is no need for extra work after the installation.

This advantage is particularly evident when dealing with largerdimensioned versions of the device and thereby leads to significant costsavings compared to welded steel constructions.

Furthermore, it is also conceivable that other materials instead of thelight metal plates are used, for example, fibre-reinforced composites orsandwich plates made of fibre-reinforced composites. Instead of screwconnections, it is also naturally possible that other distortion-freejoining techniques, such as adhesives or rivets, are employed.

In order to absorb the process-related forces, torsional forces andvibrations, the plates are, according to another preferred embodiment ofthe present invention, arranged with an optimized contour and, ifapplicable, arranged in an appropriate direction.

In such a manner, each vertical positioning unit can be optimized inrespects to weight and load.

A further advantage as compared to welded connections is that theindividual plates can be easily exchanged.

Such a vertical positioning unit made of light metal in a deviceaccording to the present invention is both economical and readilyavailable.

Another feature of the device according to the invention is thespatially static arrangement of the support frame in relation to thevertical positioning units. Especially in the case of a desired largerdimensioning of the device, it might occur that an indeterminatepositioning coincides with unfavourable circumstances, e.g. temperatureeffect or incorrect adjustment, and such could lead to enormous forcesbeing manifested and exerted at the bearing points, which could, inturn, result in deformation or even, in the worst case scenario, incollapse of the structure.

According to the present invention, it may furthermore proveadditionally advantageous that the support frame engages with thevertical positioning units by means of bearing pins and ball-jointpivoting bearings.

Preferably, it may also be provided that engagement of the bearing pinswith the ball-joint pivoting bearing is at least partially loose.

It could, for example, prove advantageous that the support frame hasball-joint pivoting bearings situated that receive the pins. On oneside, the bearing pin pair is situated loosely in the pivoting bearingsin the Y direction.

Two drillholes on the opposite side for the pivoting bearings could, forexample, be implemented in the support frame as oblong-shaped holes inthe X direction. The pivoting bearing with the pin can move in the Xdirection, thus resulting in a floating bearing arrangement of thesupport frame in the X direction.

If the frame and/or the Z axis system expands in either the X or Ydirection, then the change will be absorbed by the floating bearingarrangement. If the expansions vary, then the pivoting bearingscompensate for further angular misalignments.

If the individual Z axes move differently, then the design size-relatedlarge lever torques are also absorbed by the degrees of freedom of thesuspension points.

The ball-joint and floating bearing arrangement continues to compensatefor parallel slippage, angular slippage and linear length errors in theX and Y directions of the opposing Z axis pairs. Such errors could occurduring e.g. assembly and/or manufacture.

Moreover, a device according to the invention facilitates assembly ofthe system due to the fact that the support frame does not require exactalignment during assembly.

In the case of a permanent connection between the Z axis and the supportframe, the connection might already be damaged and/or destroyed duringassembly due to leveraging.

Furthermore, according to an embodiment of the invention, only onesingle pin needs to be inserted per Z axis and fixed in place with onesingle screw. This saves time and costs during manufacture and assembly.In so doing, the support frame becomes an easily exchangeable modularunit.

The accuracy and rigidity of the support frame can be further optimizedby utilization of highly exact plates whose contour and orientation isoptimally adjusted for the load.

A device according to the present invention can be implemented as amodular system design by employing just a few, independent and identicalmodules that can be optimally aligned. According to the presentinvention, geometric deviations due to thermal influences, errors inmanufacture, assembly and movement in the Z direction can be simply andeasily offset by a ball-joint suspension and non-locating (floating)bearing arrangement.

For the purpose of more detailed explanation, the invention is describedin further detail below on the basis of preferred embodiments withreference to the drawing.

In the drawing:

FIG. 1 a A side view of a support frame according to one preferredembodiment of the present invention;

FIG. 1 b A top view of the support frame of FIG. 1 a;

FIG. 2 a spatial representation of one preferred embodiment of thepresent invention;

FIGS. 3 a) to d) Various representations of the axle structures and theground frame according to a preferred embodiment of the presentinvention;

FIG. 4 A model of a support frame in top view for determination ofremaining degrees of freedom according to a preferred embodiment of thepresent invention;

FIG. 5 A model of the present invention according to a preferredembodiment represented in a spatial view for determination of remainingdegrees of freedom;

FIGS. 6 a) and b) A preferred embodiment of the present invention withthree vertical positioning units per respective side; and

FIGS. 7 a) and b) Another preferred embodiment of the present inventionwith four vertical positioning units per respective side, each dividedinto two modules.

As can be derived from the figures, a rectangular support frame (1) is,according to an indicated preferred embodiment of the present inventionrelating to a device for constructing three-dimensional models, made ofhollow profiles and said frame is provided to carry the positioningunits in the X direction (2). These positioning units (2) move thecoater (3) and the print head (4). This is represented in FIGS. 1 and 2as examples. What is more, the support frame (1) bears all peripheralequipment for the printing process, respectively, coating withparticulate material.

According to the invention, a support frame, such as that shown in FIG.1, may have the dimensions e.g. 5 m×3 m (length X width). With allelements mounted, the frame could have a weight of e.g. 3 metric tonnes.

The support frame (1) surrounds an area, the so-called build space, inwhich the three-dimensional object is created. During the build process,the entire support frame (1) including the peripheral equipment must bepositioned exactly in the vertical direction.

The term “peripheral equipment” hereby includes the print head (3), thecoater (4) and all components required for operation.

As shown in FIGS. 1, 2 and 3, the support frame (1) is suspended at fourpoints (two per side) located on the outer edge of the long side.

According to the displayed preferred embodiment of the invention, thesuspension points (6) of the support frame (1) are situated at itsBessel points. In this way, static flexing of the support frame (1) canbe kept to a minimum.

Each suspension point (6) is moved in the vertical direction by onesingle linear thrust unit. A vertical positioning unit consists of alinear actuator, here for example a threaded spindle and guidingelement, a drive motor (8), and an axle frame (9).

So that each vertical positioning unit moves the same distance, the fourdrive motors (8) are coupled to one another in an electronic gantrysystem.

Such an assembly of the present invention achieves a modularconstruction. Thus, according to the displayed preferred embodiment,each vertical positioning unit can be exchanged independently of oneanother without a problem.

In order to achieve high accuracy while maintaining low weight, highstability and avoidance of permanent welded connections, the axle frame(9), according to the displayed preferred embodiment of the invention,is composed of flat, milled, inherently stable light metal plates.

To obtain the required accuracy, the individually machined plates arepinned and screwed.

In order to absorb the process-related forces, torsional forces andvibrations, the plates are arranged with an optimized contour and in anappropriate direction.

The machining of the individual plates is very simple due to the factthat all areas are easily accessible and allow processing in one workstep. Material distortion due to welding or retrofitting work are ruledout since there is no need for extra work after the installation.

According to the displayed preferred embodiment of the invention, everyaxle frame structure (9) is weight and load optimized, the individuallight metal plates are easily exchangeable and a subsequent trueing ofthe construction is dispensed with. The plate construction is therebyeconomical and readily available.

According to the embodiment especially as represented in FIGS. 2 and 3,an axle structure (9) consists of five flat plates.

A guidance plate (15) bears the linear thrust unit in the Z direction.It is oriented in the X direction so that an optimal, uniform state ofstress is achieved if acceleration forces are exerted by the coater andprint head.

The two parallel support plates (16) arranged perpendicularly to theguidance plate (15) absorb forces in the Y direction. Guidance plate(15) and support plates (16) are connected with a base plate (17).Together, they support a load-bearing plate (18) from which the guidespindle (19) hangs. The force transmission proceeds here via a spindlethat hangs downward.

Alignment is obtained via two Z axle structures and/or axle frames (9)screwed in place on one side with a two-dimensional welded frame, afloor frame (7). The connection surfaces of the welded frame areprocessed in one work step. As a result, a good prerequisite foralignment of the vertical positioning units exists.

In turn, each assemblage formed by two vertical positioning units and/orthe two axle frames (9) and a floor frame (7) result in an independent,identically manufactured, space-saving and easily exchangeable module.Arranged opposite each other, two of these modules result in the entireZ axis of the device.

This assemblage is anchored to the floor via machine feet (10) that areadjustable in the X, Y and Z directions as depicted in FIG. 3. Thehighest degree of stability is thereby obtained while maintaining goodadjustability.

In order to achieve complete adjustability of every single Z axispositioning unit and/or axle frame (9), the machine feet (10) arearranged in such a way under each Z axle structure that these can beoptimally adjusted in the angle to the floor. In doing so, theindividual machine feet are adjusted variously in the Z direction.

A slight elastic deformation of the floor frame must be accepted duringalignment since the total rigidity is not changed.

Adjustability plays a major role since even the smallest manufacturingdefects of the floor frame (7) will make themselves manifest as largepositional deviations in the towering vertical positioning units.

If the vertical positioning units (9) do not stand adequately parallelin all directions, even after adjustment, then the Z linear thrust unitsand the linking of the support frame (1) will be stressed or destroyedby constraining forces, if the appropriate precautions are not takenwhen laying out the guiding elements.

The device is advantageously so designed that it is arranged in astatically determinate manner. This comes about due to application ofthe so-called Gruebler equation. This is as follows:

F=B·(n−1−g)+Σb _(i) +Σs _(i)

whereby the individual symbols mean:F: degree of freedomB: mobility type (B=6 for spatial arrangements)G: number of jointsb_(i): mobility of joint I s_(i): applied special dimensioningn: number of elements

If the arrangement of the support frame (1) is viewed according to theequation above without considering vertical mobility, then one can finda statically determinate arrangement according to FIG. 4. The supportframe (1) is hereby a combination of four pivoting bearings, forexample, the ball-joint pivoting bearings (11), each with three degreesof freedom (DOF), and the four pivots e.g. the floating bearings (12),each with two degrees of freedom fixed at the four permanent suspensionpoints.

The total number of the existing, connected construction elements equals6, the number of joints is 8.

The Gruebler equation then results in:

F=6·(6−1−8)+(4·3+4·2)=2

The two degrees of freedom relate to the rotary possibilities of theelements (20). The total arrangement is thereby statically determinateand requires no special dimensioning.

If the device is viewed in regards to the vertical positioning units,then the resulting model looks like that of FIG. 5. The verticalpositioning units are thereby modelled as purely “displacement bearings”possessing a degree of freedom.

The Gruebler equation then results in:

F=6·(10−1−12)+(4·3+4·2+4)=4

The four degrees of freedom again relate to the two rotary degrees offreedom of elements (20) as well as the rotation of the support framearound the X and Y axes.

Likewise in this case, no special dimensioning is required in order toimplement the arrangement as statically determinate. That means thateven in the case of a malfunction of the vertical positioning units,e.g. an axle breaking out of the assemblage, the support frame stillcontinues to be statically determinate and no constraining forcesresult.

According to a particularly preferred embodiment of the presentinvention, the implementation of the simplified model can proceed bymeans of connection of the support frame (1) via cylindrical bearingpins with the vertical positioning units of the Z axis system.

The ball-joint pivoting bearings (11) that receive the pins are locatedin the support frame (1). Furthermore on one side of the support frame,the bearing pins are loosely inserted in the Y direction within theirholders in the vertical positioning unit.

Preferably, the two drillholes on the opposite side for the pivotingbearings (11) are hereby implemented in the support frame (1) asoblong-shaped holes in the X direction. The pivoting bearing (11) withthe pin can move in the X direction, thus resulting in a floatingbearing arrangement of the support frame in the X direction.

If the frame and/or the Z axis system expands in either the X or Ydirection, then the change will be absorbed by the floating bearingarrangement. If the expansions are varying, then the pivoting bearingscompensate for further angular misalignments.

The ball-joint and floating bearing arrangement continues to compensatefor parallel slippage, angular slippage and linear length errors in theX and Y directions of the opposing Z axis pairs.

The cause of these errors may be attributable to assembly andmanufacturing procedures.

Moreover, the assembly of the device is enormously facilitated due tothe fact that the support frame (1) does not require exact spatialalignment during assembly.

In the case of a permanent connection between the vertical positioningunit (9) and the support frame (1), the connection might already bedamaged and/or destroyed during assembly due to leveraging.

Furthermore, only one single pin needs to be inserted per Z axis andfixed in place with one single screw. This saves time and costs duringmanufacture and assembly. In so doing, the vertical positioning unit (9)becomes an easily exchangeable modular unit.

All in all, the system as described according to one preferredembodiment of the invention is characterised by a high degree ofaccuracy and rigidity due to the fact that precisely formed plates areused, the contour and orientation of which are optimally adjusted forthe load.

A modular system design that employs just a few, independent andidentical modules makes it possible to autonomously and therebyoptimally perform alignment of the system. In addition, only minimalconstruction resources are needed to adapt the device to varyingdimensioning requirements. In this context, FIG. 6 and FIG. 7 showdesign shapes of the device according to the invention with variouslylongitudinally-dimensioned build spaces. The vertical positioning unitscan be taken over identical in construction to the original variants.The increased weight due to the larger support frame is only taken intoaccount in terms of the number and distribution of the individualmodules.

Moreover, the production environment for such device does not place anybeyond-the-normal demands insofar as the foundation since forces can beoptimally transferred into the floor. This can be accomplished by afixed connection of the Z axis with the floor via machine feet (10) thatare adjustable in the X, Y and Z directions.

An advantageous feature of the present invention is that geometricdeviations due to thermal influences, errors in manufacture, assemblyand movement in the Z direction can be simply and easily offset by aball-joint suspension and non-locating (floating) bearing arrangement.

DESIGNATION LIST

-   -   1 Support frame    -   2 Positioning units in X direction    -   3 Coater    -   4 Print head    -   6 Suspension points    -   7 Floor frame    -   8 Drive motor    -   9 Axle frame, positioning unit in Z direction    -   10 Machine feet    -   11 Pivoting bearings    -   12 Non-locating (floating) bearings    -   15 Guidance plate    -   16 Support plate    -   17 Base plate    -   18 Load-bearing plate    -   19 Guide spindle    -   20 Machine part with rotary degree of freedom

What is claimed is:
 1. A method for assembling an apparatus formanufacturing three-dimensional models by means of a 3D printing processcomprising: arranging at least i) a first pair of vertical positioningunits and ii) a second pair of vertical positioning units on a commonsubstrate, wherein each vertical positioning unit has a drive direction;independently adjusting each of the vertical positioning units so thatthe drive direction of each vertical positioning unit is in a verticaldirection; attaching the first pair of vertical positioning units to afirst side of a support frame; and attaching the second pair of verticalpositioning units to a second side of the support frame, wherein thefirst and second sides are opposing sides; wherein the support frame isarranged around a build platform having a base surface for building the3-D model; the support frame is moveable in a vertical direction by thevertical positioning units; and the base surface of the build platformis perpendicular to the vertical direction. 2-11. (canceled)
 12. Themethod of claim 1, wherein the support frame supports a particulatematerial dosing device.
 13. The method of claim 12, wherein the methodincludes moving the particulate material dosing device in an X-directionvia guiding elements for layerwise depositing of particulate material.14. The method of claim 12, wherein the support frame supports a devicefor feeding a bonding material.
 15. The method of claim 14, wherein themethod includes moving the device for feeding a bonding material in anX-direction for depositing the bonding material.
 16. The method of claim1, wherein the first pair of vertical position units are placed along afirst line and the second parallel vertical positioning units are placedalong a second line, wherein the first line and the second line areparallel.
 17. The method of claim 16, wherein three vertical positioningunits including the first pair of vertical positioning units are placedalong the first line and three vertical positioning units including thesecond pair of vertical positioning units are placed along the secondline.
 18. The method of claim 1, wherein each vertical positioning unithas at least three vertically adjustable feet.
 19. The method of claim15, wherein each vertical positioning unit has at least three verticallyadjustable feet.
 20. The method of claim 1, wherein the supportingstructure is flexibly deformable.
 21. The method of claim 1, wherein themethod includes moving the support frame in the vertical direction. 22.The method of claim 1, wherein each of the vertical positioning unitsincludes a drive motor.
 23. The method of claim 22, wherein the methodthe drive units are coupled with each other via an electronic gantrysystem.
 24. The method of claim 21, wherein the method includes movingthe support frame in the vertical direction after applying a layer of abuild material to the build platform.
 25. The method of claim 1, whereinthe first pair of vertical positioning units are positioned in spacedapart locations on a first linear track and the second pair of verticalpositioning units are positioned in spaced apart locations on a secondlinear track.
 26. The method of claim 25, wherein the first linear trackis angled relative to a horizontal plane and/or the second linear trackis angled relative to a horizontal plane.
 27. The method of claim 1,wherein the common substrate is not flat.
 28. The method of claim 1,wherein every vertical positioning unit engages with the support frameat a suspension point and is independently moveable in the verticaldirection.
 29. The method of claim 1, wherein the method includes movingone of the vertical position units in the vertical directionindependently from the other vertical positioning units.
 30. The methodof claim 1, wherein the support frame engages with the verticalpositioning units by means of bearing pins and ball-joint pivotingbearings.